The Roman army had a thousand years of tradition behind it by 284, the accession of Diocletian. He took over an army that for three-quarters of a century had failed to keep Germans and Persians out of the empire but succeeded in killing all but three of his twenty-seven predecessors.
The fundamental problem was that four different sectors of the frontier constantly needed defending: the East, threatened by the Persian Empire, and the Lower Danube, Upper Danube, and Rhine rivers, all threatened by German tribes. Large armies under responsible commanders had to be stationed at all four trouble spots. Since the emperor could only be in one sector at a time, the generals in each of the other three had to be left an army that was large enough to repel the enemy, which unfortunately was also large enough to support a rebellion. When a general’s troops proclaimed him emperor, he would march away from the frontier, letting in the barbarians.
Diocletian worked hard to make the army less prone to rebel and better at defeating the enemy. To make the army stronger, he increased its size. Though the amount of the increase is controversial, Diocletian certainly enforced a strict system of conscription. He required soldiers’ sons to enlist, and demanded that taxpayers either produce recruits or pay for bounties to attract them. To make the army more contented and efficient, Diocletian reguralized its pay. This consisted largely of food, arms, and uniforms supplied in kind.
Perhaps most important, to give each weak point in the frontier a capable commander who would not proclaim himself emperor, by 293 Diocletian chose three trusted generals and proclaimed them emperors himself. He became senior emperor in the East, and after some shifting of responsibilities took over the eastern frontier while his junior emperor Galerius guarded the Lower Danube. In the West another senior emperor – though not quite as senior as Diocletian – held the Upper Danube with a junior emperor to hold the Rhine. Today this system of two senior and two junior emperors is often called the tetrarchy, though Diocletian considered the main division to be twofold, between East and West.
Each of the four emperors commanded the soldiers in his sector, most of whom were stationed along the frontiers. Diocletian grouped them into a chain of regional commands under dukes (duces), who were independent of the provincial governors and sometimes defended two or three small provinces. The dukes commanded forces of infantry legions and other cavalry and infantry units, and the dukes along the river frontiers had fleets. The emperors, who also kept small mobile reserves wherever they were, mustered the dukes’ troops when they were needed for campaigns.
For some years the system worked remarkably well. Diocletian and his colleagues defeated the Persians, the Germans, and whatever rebels appeared, and secured the frontiers and internal order for the first time anyone could remember. In 299 Diocletian even annexed some border territory from Persia; though his intention was evidently to punish Persian aggression rather than to expand, this was the empire’s first foreign conquest in a hundred years. In 305 Diocletian abdicated voluntarily, allowing his subordinate Galerius to become senior emperor of the East and to choose a junior colleague, Maximin. In this way the tetrarchy was supposed to renew itself indefinitely.
Yet without Diocletian’s restraining influence, civil war broke out in the West within a year of his abdication. In 307 Galerius led an army into Italy to restore order, but he had to withdraw when many of his men deserted to the rebel western emperor Maxentius. Several years of fighting and intrigue left both East and West split between hostile emperors. In 312 Constantine I, the emperor of the Rhine, eliminated Maxentius, the emperor based at Rome and in charge of the Upper Danube. The next year Galerius’ successor Licinius, the emperor for the Persian frontier. Constantine took the Lower Danube sector from Licinius in 317, and completed his conquest of the empire by defeating his rival in 324. He founded Constantinople near the site of his final victory in a naval battle.
Having disbanded the old Praetorian Guard of the Roman emperors, Constantine created a new cavalry corps known as the Scholae as his own guardsmen and agents. During his conquests Constantine had assembled a sizable field army, probably drawn in large part from the mobile reserves of his rivals. He kept it distinct from the frontier troops, making it a standing force of infantry and cavalry that was to accompany the emperor wherever he went. But Constantine kept separate administrations for the four parts of the empire. These he entrusted to his three surviving sons and a nephew, who inherited them at his death in 337.
Constantine’s son Constantius II held the eastern frontier, and divided the Lower Danube with his brother Constans I after the troops lynched its intended ruler, Constantine’s nephew. The three brothers took over the frontier forces in their domains and divided the field army, which they put under masters of soldiers (magistri militum). At first each emperor had one master of soldiers for cavalry and one for infantry; but emperors soon began to appoint separate masters of soldiers for the field forces deployed in the provinces and for those kept “in the Emperor’s Presence”, the praesental army (praesentales). Besides his praesental army, Constantius maintained a field army of the East to watch the Persian frontier, and after the assassination of his brother Constans in 350 Constantius kept another field army in Illyricum on the Lower Danube.
The army defended the East rather well until the emperor Julian arrived from the West after his cousin Constantius’ death and invaded the Persian Empire in 363. Julian’s advance to the Persian capital of Ctesiphon accomplished nothing; Julian died of a wound, and the expedition ran out of supplies. The guardsman whom the army chose as Julian’s successor brought his men out safely by agreeing to some cessions of border territory to the Persians. This failure was more Julian’s fault than that of the army, which emerged with limited losses.
The next year, when the throne fell vacant again, the army chose another bodyguard, Valentinian I. He decided to become emperor of the West, and named his brother Valens emperor of the East. After the brothers divided the field forces, Valens deployed his share in three armies under masters of soldiers: one praesental, one for the East, and a third for Thrace, on his part of the Lower Danube frontier. The rest of the Lower Danube was Valentinian’s, under a Master of Soldiers for Illyricum.
While the field armies gained in importance, the frontier forces became second-class troops. These troops were usually adequate to deal with bandits and rioters, but they were less good at fighting foreign enemies. This came to be a problem, because the Huns had appeared to the northeast, frightening the Goths, the Germans settled across the Lower Danbe, into seeking refuge in imperial territory.
In 378 the temporarily united Goths inflicted a crushing defeat on the eastern field armies near Adrianople in Thrace, killing Valens and many of his men. This chaotic battle, caused by various Roman mistakes, seems not to have resulted from any basic weakness in the army beforehand, but it certainly caused weaknes afterward. To save the situation, the western emperor Gratian, Valentinian’s son, chose the capable general Theodosius as eastern emperor. Besides Valens’ territories, Theodosius I received most of Illyricum, not only because the Goths threatened it but because its army, having suffered no losses at Adrianople, was the empire’s only effective force in the region.
To replace the losses at Adrianople, Theodosius feverishly recruited new soldiers, including many Germans, some of them deserters from the Goths, and other barbarians. Ending the practice of appointing separate commanders for infantry and cavalry, he united the field armies of Illyricum, Thrace, the East, and the Emperor’s Presence, each under a single master of soldiers. With reinforcements from Gratian, Theodosius managed to keep the Goths more or less confined to northern Thrace. Their offshoots, the Visigoths and Ostrogoths, eventually agreed to make peace with the empire. The Ostrogoths settled in Gratian’s part of Illyricum and the Visigoths in Thrace, officially as Roman allies but with almost complete independence.
Yet Theodosius gained time to continue rebuilding the army. Even without the Army of Illyricum, which he returned to Gratian, he was able to defeat Ostrogoth raiders in 386 and Gratian’s murderer Maximus in 389. Though Theodosius then established Gratian’s brother Valentinian II in the West, he kept the Army of Illyricum for himself. He also transferred some western field troops to the East in 388, and with them expanded the praesental army into two praesental armies of similar organization. In 394 Theodosius won another war, this time with western rebels who had murdered Valentinian II. The next year Theodosius died, dividing a somewhat strengthened empire between his two sons, Arcadius in the East and Honorius in the West.
Theodosius had taken most of the eastern field army with him to the West; on his death, it was to be returned to the East. While the advisers of his two young sons quarreled with each other, the eastern field army did return, to murder Arcadius’ adviser Rufinus. Arcadius’ new adviser Eutropius then reassembled the field armies of Illyricum, Thrace, the East, and the two praesental armies. The organization of the eastern forces at this time is largely known from the Notitia Dignitatum, a comprehensive list of the empire’s principal officers and officials. The portion for the eastern empire dates to about 395; the portion for the West is rather later. The eastern Notitita catalogues the legions and other units belonging to the East’s five field armies and fifteen ducates of border troops.
Though the organization of the eastern army scarcely changed throughout the fifth century, this was a far from easy time for either the army or the Eastern Romans. The Visigoths ravaged Illyricum for several years until they left for the western empire. In 400 Gainas, a Visigoth Master of Soldiers in the Emperor’s Presence, briefly took over Arcadius’ government in Constantinople, before being driven out and killed. The Huns repeatedly raided Thrace and Illyricum, forcing the Eastern Romans to pay them tribute and to fortify Constantinople with an almost impregnable wall. The Eastern Romans had temporarily to evacuate much of the Danube frontier, where the Huns raided at will until they turned to attacking the western empire in 450.
While the eastern emperors happened to be weak rulers, Germans and other barbarians remained strong in the armies, and Aspar, an Alan Master of Soldiers in the Emperor’s Presence, was virtual ruler of the East by 457. To balance the influence of Aspar and his Germans, the eastern emperor Leo I recruited many Isaurians, warlike mountaineers from southeast Anatolia. The Isaurian leader Zeno first became head of the imperial guard, which included the newly created corps of Excubitors. Then Leo named Zeno Master of Soldiers for Thrace, and made him presumptive heir to the throne by a marriage to the emperor’s daughter.
In 468 Leo launched the one great eastern offensive of the century, a joint expedition with the West to reconquer North Africa from the Vandals. Its success would probably have saved the West. But the campaign was a catastrophic failure, which doomed the West and left the East seriously weakened. Apparently with some reason, many blamed the defeat on treachery by barbarian officers in both the western and the eastern armies, and by Aspar in particular. Three years later Leo managed to murder Aspar and to replace him with Zeno.
But Aspar’s murder enraged the many Ostrogoths in the army. They joined the Ostrogoths still settled in Illyricum in ravaging Thrace, and had to be bought off. When Zeno succeeded to the throne in 474 he inherited a serious Ostrogoth problem. He only solved it, after years of fighting, by sending the Ostrogoths to Italy in 488, supposedly to punish the Germans who had recently overthrown the last western emperor. Perhaps to limit the number of barbarians in the army, and in any case to control them, Zeno legislated that all recruitment should be monitored by the central government.
While the tetrarchy had given rise to civil wars among its members and their heirs, no one from outside the system ever broke into it more than momentarily in the East; after Constantine no eastern rebel of any sort did so. In the East, though not in the West, the changes Diocletian made in the army achieved more or less what he had intended.
(Source: “Byzantium and its Army, 284-1081”, by Warren Treadgold)
Research-Selection for NovoScriptorium: Anastasius Philoponus